Data communication systems are frequently described in terms of a “protocol stack”, which groups the sequences of tasks required to operate the system into logically related groups known as “layers”. Conceptually, higher layers have a higher level of abstraction; e.g. the user applications are at the highest layer while the circuits responsible for transmitting the data e.g. over the air or over a copper wire are at the lowest layer.
An example of this is the 802.11 wireless LAN (WLAN) standard, as represented in simplified form in FIG. 1. The 802.11 protocol stack is divided into the MAC (medium access control) layer and PHY (physical) layer. When transmitting, the MAC layer takes messages from the layer above, appends addressing and error-checking information, checks that the wireless medium is free, and passes the expanded message to the PHY layer. The PHY layer formats the data for transmission, adds PHY-specific information (e.g. a preamble and transmission rate information), modulates the data and transmits it onto the antenna. At the receiver, the PHY layer receives the transmitted data, using the PHY-specific information, and passes the MAC-level message to the MAC layer. Here, the received message is checked for errors, and if the message is addressed to the device in question the data is passed up to the higher layer.
One of the benefits of this logical organization into separate layers is that functions specific to an individual layer can be added or enhanced while retaining compatibility with other layers in the system, and different physical layers can be implemented. For instance, the original 802.11 WLAN standard defines PHY layers operating over radio or infrared links, while the 802.11a enhancement to the 802.11 standard offers a higher rate of data transmission (up to 54 Mbp/s) over a radio link. At the MAC layer, the 802.11e draft standard offers a number of enhancements to the basic 802.11 MAC protocol to support better throughput, better scheduling data delivery and enhanced protection against transmission errors.
While the separation into protocol layers has great benefits in terms of logical structure and extensibility, certain disadvantages and problems can occur due to interactions between functions in different layers. An example of this occurs when 802.11e MAC level forward error control is applied over a link using the 802.11a high-rate PHY.
The conventional 802.11 MAC layer prepends a 32-octet MAC header to the message sequence, containing addressing and control information, and appends a 4-octet “frame check sequence” to the message which is a 32-bit cyclic redundancy check value that can be used to detect almost all possible errors in the transmitted data. This structure is outlined in FIGS. 2a and 2b. 
The forward error control (FEC) system used in the 802.11e draft standard is based on Reed Solomon coding, breaking the transmitted message into blocks of 208 or fewer octets to each of which a 16 octet error control code is appended. This is done in such a way that the resulting frame appears as a conventional 802.11 MAC frame to non-802.11e aware devices: all FEC information is contained within the frame body, and the MAC header and FCS are prepended and appended as for standard 802.11, allowing interpretation of address information and checking for correct transmission. Within the frame body, a 16-octet FEC field is added to protect the MAC header, while the data is split into blocks of 208 octets protected by a 16-octet FEC field. A frame check sequence calculated over the MAC header and message is appended to the final block of data. This inner “FEC FCS” can be used for final confirmation that error correction was successfully able to correct for errors introduced in transmission. The code used is capable of correcting up to 8 octets in each block of 224 data and FEC octets, and thereby offers reasonably strong protection against transmission errors.
The 802.11a high-rate PHY layer offers data rates of up to 54 Mbps operating in the 5 GRz radio band. The 802.11 g draft PHY standard uses an essentially identical modulation format in the 2.4 GHz radio band, and so the issues discussed here apply to the majority of new IEEE 802.11 WLAN equipment when trying to benefit from MAC-level FEC.
The 802.11a PHY layer takes the MAC-layer frame, and performs scrambling on the data in order to make the characteristics of the transmitted modulated sequence independent of the message being transmitted. The scrambler circuit specified in the 802.11a standard is shown in FIG. 3 and is made up of a linear feedback shift register (LFSR) whose output is XOR:ed with the incoming data. The sequence generated is uniquely defined by the initial state of the delay elements D1–D7, which is known as the seed value for the scrambler. The standard defines that this seed value should be set to a pseudo-random non-zero state for each message transmitted.
At the receiver, the same seed value must be loaded into the delay elements. The same sequence can then be generated at the receiver and XOR:ed with the incoming data stream, thereby recovering the original data. For this to be possible, a sequence of 7 zero bits is prepended to the message (followed by 9 bits whose use is reserved for future supplements to the standard). The whole prepended 16-bit field is called the service field. Since the original data is known to be zero for the initial 7 bits, it is possible to deduce the initial state of the scrambler from the transmitted sequence.
Clearly, correct function of the design is dependent on successfully receiving these 7 bits so as to be able to initialize the scrambler correctly. If the scrambler is incorrectly initialized, the entire subsequent message will be corrupted since the wrong sequence will be generated. When the original 802.11a standard was formulated, there was no error correction proposed in the MAC layer, so this propagation of errors was unimportant: any error would mean that the message would be discarded. However, when trying to implement MAC-level error correction, this error propagation severely limits the level of error correction that is possible at medium to high signal to noise ratios. Fundamentally, the probability that a frame must be discarded becomes dominated by the probability of having one or more bit errors in the 7-bit scrambler initialization sequence rather than the probability of having an error in the data that cannot be corrected.
These differences are shown graphically in FIG. 4, where the unbroken line shows the probability of more than 8 octet errors occurring in a given block of 224 (i.e. the probability of an FEC failure) for a given underlying bit error rate, on the assumption that bit errors are independent of one another. The dashed line shows the probability of one or more errors occurring in the scrambler initialization field (the probability of a scrambler failure). It is clear that at underlying bit error rates higher than approximately 10−2,7, the chance of the FEC failing (more than 8 octet errors in any given 224 octet FEC block) is greater than the chance of an error in the 7 bits of the scrambler error. However, when the underlying bit error rate decreases, the theoretical resulting packet error rate with FEC should rapidly become very small, but this does not happen due to the error propagation problem with the descrambler.
In typical applications where MAC level forward error correction is desired, such as distribution of audio and video data streams, the tolerable bit error rate is generally low. The difference between the theoretical performance from the FEC system and the limit caused by scrambler error propagation means that more transmit power or lower range must be accepted to maintain a given level of performance in these cases.
In order to be valid within the scope of the 802.11e MAC layer draft standard, any solution to the problem must be fully compatible with other devices that implement the 802.11a PHY standard. It is also desirable that the 802.11 MAC header and FCS field can be interpreted by a non-802.11e aware device, which is a property maintained in the current 802.11 e FEC MAC frame structure.
One known solution is to calculate error control bits on the scrambler initialization bits (e.g. adding 4 bits by using a [11,7] Hamming code), with the bits transmitted in the reserved section of the PHY service field.
A second solution is to transmit the service field (scrambler initialization data) at a lower data rate that is less susceptible to errors, while switching to a higher data rate for the message part of the packet.
There are however some problems associated with the above described solutions.
The first solution requires the use of bits specified as reserved in the 802.11a PHY layer standard, and thereby requires a change to the standard.
The second solution also requires a change to the 802.11a PHY layer standard, since it is not currently supported to transmit the service field at a different transmission speed.
Thus there is a need for a method for reducing or limiting the transmission errors due to incorrectly transmitted seed values that is valid within the present standard.